Published German patent application DE-OS 4,102,376 discloses a transmitted-light refractometer which includes a so-called differential cuvette having two prism-shaped chambers. One of the chambers receives the sample liquid and the other chamber receives the reference liquid. The partition between the two chambers is provided by a partition wall inside the cuvette. This partition wall is arranged so as to be inclined to the optical axis.
German patent publication DD 254,069 discloses a transmitted-light refractometer having a cuvette with only a single prismatic chamber. The refractive index measurement is made here without a reference liquid simply with the aid of the beam deflection within the prismatic-shaped sample volume.
The known transmitted-light refractometers all have in common that the refractive index of the sample is determined with the aid of the deflection of the measurement light effected by the prismatic sample volume relative to the deflection registered for a previously conducted calibration measurement with a liquid of known refractive index. A disadjustment of the illumination and viewing optics which occurs in the meantime leads to measurement errors.
Published German patent application DE-OS 4,038,123 discloses a differential refractometer wherein the cuvette is partitioned by a V-shaped wall into a measurement chamber and a reference chamber so that two slit images occur on the line sensor. The volume or cross section available for the through-flow measurement is very small for the sample because of this partition of the cuvette.
Furthermore, so-called total refractometers are known wherein measurement light incidents upon the probe at different angles containing the critical angle of total reflection. The refractive index is determined with the aid of the critical angle of total reflection. One such refractometer is described in European patent publication 0,184,911. Also here, the disadjustments of the optical components occurring after the calibration measurement lead to measurement errors. A further disadvantage is that the measurement result is determined only from an edge layer of the sample which is very thin. This edge layer is, however, generally (for example, for laminar disturbances) not representative of the entire sample. Deposits or small bubbles in the edge layer therefore act to falsify the measurement result.